Several arenaviruses cause hemorrhagic fever (HF) disease in humans and pose a serious public health problem in their endemic regions. Thus, the Old World arenavirus (OWA) Lassa (LASV) infects several hundred thousand individuals yearly in West Africa resulting in a high number of Lassa fever (LF) cases associated with high morbidity and mortality. Moreover, evidence indicates that the worldwide-distributed OWA LCMV is a neglected clinically important human pathogen. In addition, several arenaviruses including LASV and LCMV pose a credible biodefense threat. Existing anti-arenaviral therapy is limited to an off-label use of ribavirin that is only partially effective, and there are no FDA-licensed, or currently in clinical trials, arenavirus vaccines. However, the MOPV/LASV reassortant (ML29) is a candidate live-attenuated vaccine (L-AttV) for LASV that has shown promising results in animal models. Nevertheless, as with many other traditional L-AttV, the mechanism of ML29 attenuation remains unknown, which raises concerns about the phenotypic stability of ML29 in response to additional mutations. The central goal of this application is to test the hypothesis that we can convert a pathogenic OWA into an attenuated form with features of L-AttV via replacement of its L IGR by a genetically defined synthetic S-like IGR (Ssyn). Our hypothesis is supported by our recent following findings: 1) We could rescued rLCMV(IGR/S-S) with the same S-IGR in both S and L genome segments, and found that it was highly attenuated in vivo. 2) Mice immunized with rLCMV(IGR/S-S) were protected against a lethal challenge with wild type (WT) LCMV. 3) We have obtained evidence that a high degree of sequence plasticity within the S IGR is compatible with virus viability, which supports the feasibility of engineering recombinant arenaviruses with a synthetic S IGR (Ssyn) in the L segment as a general molecular strategy for arenavirus attenuation. To test our hypothesis we propose to complete the following specific aims: Aim 1. Characterize rLCMV (IGR/S-Ssyn): We have rescued rLCMV(IGR/S-Ssyn) where a synthetic S IGR (Ssyn) substituted for the L-IGR. We will characterize rLCMV(IGR/S-Ssyn) in cultured cells and using the mouse model of LCMV infection test the hypothesis that rLCMV(IGR/S-Syn) displays key features for L-AttV. Aim 2: Generate and characterize rLASV(IGR/S-Ssyn): We will generate rLASV(IGR/S-Ssyn) and test the hypothesis that it is attenuated in vivo but able to induce protective immunity against a letha challenge with WT LASV in a well-established guinea pig model of LASV infection. Aim 3. Examine the stability of rLCMV(IGR/S-Ssyn): We will test the hypothesis that rLCMV(IGR/S-Ssyn) is genetically stable during multiplication under different growth conditions. The successful completion of this application will uncover a general molecular strategy for arenavirus attenuation that can facilitate a novel strategy to develop L-AttV to combat human pathogenic arenaviruses.